How the eye sees
Light-sensing cells in each eye
Roughly 100 to 120 million rods and about 6 million cones
Rods for dim light and the edges of vision; cones for color and detail.
Layers in the retina
Ten
Light crosses the inner layers before it reaches the rods and cones at the back.
Kinds of cone
Three
Tuned to short, medium and long wavelengths; together they give color vision.
Time for rods to adapt to the dark
About 40 minutes
Why a dark room stays dark for a while, and why night vision is the first thing some conditions take.
Light's journey through the eye
Six stops, front to back. The first four bend and admit light; the last two turn it into a signal and carry it away.
The eye from above, simplified. Light (orange) enters at the left and is focused on the fovea, the center of the retina. Schematic, not to scale.
Cornea
The clear window at the front. It does most of the eye's focusing before light reaches the lens.
Pupil and iris
The iris is the colored ring; the pupil is the opening in it. Together they set how much light gets in.
Lens
Fine-tunes the focus by changing shape, so near and far objects can both be sharp.
Vitreous
The clear gel that fills the eye and keeps its shape. Light crosses it untouched.
Retina
The thin layer lining the back of the eye, where light becomes a nerve signal. Its center, the macula, holds the fovea, the point of sharpest sight.
Optic nerve
About 1.2 million nerve fibers bundled together, carrying the retina's signal to the brain.
The retina, where seeing begins
The retina is built back to front: light passes through the signal-carrying layers before it reaches the rods and cones, which sit against their support layer at the very back. Inherited retinal diseases live in these last two layers.
Five bands stand in for the retina's ten layers. Light arrives from the top and travels down to the rods and cones. Schematic.
Rods
About 95% of the photoreceptors
What they do
See in dim light, and cover the wide outer retina that gives you side vision. They see in shades of gray, not color.
When they fail
Night vision goes first. In retinitis pigmentosa, the most common early symptom is loss of night vision, followed by narrowing side vision.
Cones
About 6 million, packed into the macula
What they do
See color and fine detail in good light. The fovea, at the center of the macula, is cones only.
When they fail
Central vision goes: reading, driving, faces. Stargardt disease, for example, affects the macula.
The quiet layer behind them
The retinal pigment epithelium is a single sheet of cells behind the rods and cones. It recycles vitamin A so the photoreceptors can keep responding to light, clears the worn tips they shed, and forms part of the barrier between the retina and its blood supply. RPE65, the gene behind the one approved gene therapy, does its work in this layer.
From the eye to the brain
Seeing finishes at the back of the head. The signal passes six stations on the way; inherited retinal disease acts at the first one.
Retina
Rods and cones turn light into a signal; the retina's own cells sharpen it.
Optic nerve
The ganglion-cell fibers from one eye, bundled together.
Optic chiasm
Fibers from the inner half of each retina cross to the other side, so each half of the brain sees the opposite half of the world.
Lateral geniculate nucleus
A relay in the thalamus that sorts the signal.
Optic radiations
Fibers fanning back through the brain.
Visual cortex
At the back of the brain, where the signal becomes sight.
Inherited retinal diseases are diseases of the retina, the first station. That is why the treatments on our
therapy guide
are aimed at the eye.
Color, field and light
Three things the retina gives you, and how retinal disease changes each.
Color
Three kinds of cone, tuned to short, medium and long wavelengths (roughly blue, green and red light), mix to give every color you see. In achromatopsia, which affects about 1 in 30,000 people, the cones don't work: color vision is absent, light is uncomfortably bright, and detail is poor.
Field of view
Detail comes from the fovea; everything to the sides comes from the wide outer retina. Retinitis pigmentosa narrows side vision, so things at the corners of your eyes disappear first. Conditions of the macula do the opposite, taking the center and leaving the edges.
Light and dark
Cones work in daylight; rods take over in dim light. Switching from bright to dark is slow: rods need about 40 minutes to reach full sensitivity, which is why a dark room keeps getting clearer. When rods are failing, dim places never clear. Loss of night vision is the most common early symptom of retinitis pigmentosa, often starting in childhood.
Side vision lost
Center lost
What inherited retinal disease changes
Which cells a condition affects decides what changes first. The same four cell groups come up again and again.
Where the disease acts
What usually changes first
Examples
Rods
Night vision, then side vision
Retinitis pigmentosa; Usher syndrome (retinitis pigmentosa with hearing loss)
Cones
Color, detail and comfort in bright light
Achromatopsia; cone dystrophy
The macula
Central vision: reading, faces
Stargardt disease; Best disease
The retinal pigment epithelium
Depends on the gene; some act from infancy
RPE65-related retinal dystrophy
Many conditions affect more than one group, and the order varies from person to person. Your condition's own page has the specifics.
Sources
StatPearls, Neuroanatomy, Retina (NCBI Bookshelf): ten retinal layers; about 6 to 7 million cones and 95% rods; the fovea is cones only; RPE functions; about 40 minutes to restore full rod function in darkness.
American Academy of Ophthalmology, Cones (eye-health anatomy page): approximately 120 million rods and 6 million cones.
StatPearls, Color Vision (NCBI Bookshelf): three cone types and their peak wavelengths; about 100 million rods.
StatPearls, Neuroanatomy, Visual Pathway (NCBI Bookshelf): the sequence from optic nerve to occipital lobe; nasal fibers cross at the chiasm; about 1.2 million optic nerve fibers.
National Eye Institute, Retinitis Pigmentosa: loss of night vision as the most common early symptom; loss of side vision.
MedlinePlus Genetics, Achromatopsia (about 1 in 30,000; color vision, photophobia, low acuity) and Stargardt macular degeneration (affects the macula; reading, driving, recognizing faces).